%%
%% The LaTeX Companion, 3ed
%%
%% Example A-1-10 on page II-632 in "Defining simple environments".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{151.0pt}
%StartShownPreambleCommands
\usepackage{calc}
\newenvironment{altDescription}[1][\quad]%
  {\begin{list}{}{%
   \renewcommand\makelabel[1]{\hfil\textsf{##1}}%
   \settowidth\labelwidth{\makelabel{#1}}%
   \setlength\leftmargin{\labelwidth+\labelsep}}}
  {\end{list}}
%StopShownPreambleCommands

\begin{document}
\begin{altDescription}
\item[First] This is a short term with text that wraps.
\item[Long term] This is a long term.
\item[Even longer term] A very long term.
\end{altDescription}
\begin{altDescription}[Even longer term]
\item[First] This is a short term with text that wraps.
\item[Long term] This is a long term.
\item[Even longer term] A very long term.
\end{altDescription}
\end{document}
